"Bill Gross had been synonymous with this fund since its 1987 inception, so his September 26, 2014, resignation was jarring," said Eric Jacobson, senior analyst at Morningstar.

Jacobson said that while Gross had never denied having a "tremendous amount of help" running the world's largest bond fund, his public profile gave the impression he had "single-handedly built the fund's long-term record."

"Morningstar remains positive overall on Pimco Total Return after the departure of Bill Gross, but is downgrading the fund to bronze because of the resulting uncertainty regarding outflows and the reshuffling of management responsibilities," Jacobson said.

The fund's new rating reflects Morningstar's "high level of confidence" in Pimco's resources, but also uncertainty as to how exactly management and resources will "mesh in the wake of Gross' departure."

Jacobson said that in the short term, the $222 billion fund appeared well-positioned to "weather a pretty large storm," despite some "choppy" recent performance.

"It's possible that outflows following Gross' departure could be much worse, but the period from the fund's May 2013 asset peak to September 2014 saw roughly $70 billion in outflows according to Pimco, and there has been no indication that they caused any unusual problems," said Jacobson.

Deutsche Bank has forecast that investors will pull around 210 billion euros ($266 billion) from Pimco overall over the next two years.

Gross, a renowned investor often referred to as the "bond king," quit Pimco on Friday, citing "fundamental differences" with the company he co-founded 40 years ago.

Gross has taken a new post at Janus Capital, where his arrival sent shares in the fund management company surging.

"I chose Janus as my next home because of my long-standing relationship with, and respect for, chief executive officer Dick Weil and my desire to get back to spending the bulk of my day managing client assets," Gross said in statement last week.